1. Field of the Invention
The present invention relates to a novel infrared-absorptive compound and an optical recording medium making use of the infrared-absorptive compound.
2. Related Background Art
Generally, information can be recorded on optical recording mediums such as optical disks and optical cards by forming optically detectable minute pits (with size of, for example, about 1 .mu.m) so arranged as to provide a spiral, concentric or linear track or tracks on a recording layer formed on a substrate of the recording medium, thereby making it possible to store information at a high density.
As describe, for example, in "Review and Analysis of Optical Recording Media", Optical Engineering, Vol. 15, No. 2, March-April 1976, p. 99 ff., a known optical recording method comprises applying a light beam such as a laser beam to the recording layer of an optical recording medium so as to generate deformation or pits on the recording layer. In other known methods, the application of such a light beam causes the generation of bubbles, phase changes, discoloration, discolorization or the like.
Various materials have been proposed for the recording layer of such optical recording mediums. Examples of the materials include inorganic materials such as a metal film comprised of an aluminum deposit film or the like, a bismuth film, a tellurium oxide film and a chalcogenite type non-crystalline glass film. Such thin films are commonly sensitive to light having a wavelength of approximately 350 to 800 nm and exhibit a high reflectance to laser beams. One disadvantage of these films is that they provide a rather poor laser beam ultilization factor. Moreover, such inorganic material thin films are usually formed as recording layers by sputtering or the like. Such a film forming method, however, requires a production line equipped with a vacuum system, so that these inorganic material thin films are involved in a higher production cost.
In view of the above problems, optical recording mediums using coloring matter whose optical properties can be changed by the energy of light having a relatively long wavelength (for example, 780 nm or more) are being extensively studied. Optical recording mediums using such organic coloring matter are effective in that they allow the formation-of pits by a semiconductor laser having an oscillation wavelength of around 780 nm or 830 nm. Organic coloring matter also allows film formation by wet coating and can be handled with ease, making it possible to readily mass-produce recording mediums at less equipment cost.
The optical recording mediums making use of organic coloring matter in the recording layer, however, commonly have a poor light-resistance stability. For example, when such optical recording mediums are left under natural light or repeatedly irradiated with reproducing light, there has been the problem that the recording-reproducing performance of the optical recording mediums is lowered. To cope with such a problem, a method in which a nickel chelate complex serving as a singlet oxygen quencher is added to a recording layer is known in the art (for example, Japanese Patent Applications Laid-open No. 63-1594, No. 63-9576, No. 63-31792, No. 63-67187, No. 63-78794 and No. 63-82789).
The nickel chelate complex compound, however, has so poor a solvent solubility that it has been difficult for it to be added in an amount large enough to well improve the light-resistance of the recording layer when the recording layer is formed by wet coating.
In the meantime, the present applicants have disclosed in U.S. Pat. Nos. 4,656,121 and 4,923,390 a technique in which an infrared-absorptive compound such as a triallylamine type aminium salt compound or diimonium salt compound is added to the recording layer in order to improve light-resistance stability of the organic coloring matter recording layer. The aminium salt compound and diimonium salt compound have a better solvent solubility than the nickel chelate complex and are effective for well improving light-resistance stability of the recording layer.
Incidentally, temperatures inside apparatus tend to rise as recording-reproducing apparatus for optical recording mediums have been made small-sized in recent years. In addition, optical recording mediums such as optical cards, of the type they are carried by personal users, are not necessarily used only in offices well air-conditioned. For example, even a condition in which they are left for a long time in an environment of high temperature as in rooms of automobiles must be taken into consideration. Thus, there is a demand for optical recording mediums having much better storage stability in the environment of high temperature. With such a demand, the aminium salt compound or diimonium salt compound itself which is added to the recording layer in order to improve light-resistance stability as stated above is sought to have much more improved heat-resistance while retaining the solvent solubility.